Two hundred and seventy patients will be randomized to receive a single measles vaccination (Measles, Mumps, and Rubella Virus Vaccdine Live [MMR-II]) at 12 months of age or receive two measles vaccinations, one at six months (Measles Virus Live [Attenuvax]) and one at 12 months of age (MMR-II). Antibody levels will be measured to evaluate patient response.